1. Field of the Invention
The present invention relates to zoom lenses, and particularly, though not exclusively, relates to an ultra-compact zoom lens.
2. Description of the Related Art
One of the frequently used wide-angle zoom lenses is a three-unit zoom lens including, in order from an object side to an image side, a lens unit with a negative refractive power, a lens unit with a positive refractive power, and another lens unit with a negative refractive power. The three lens units are moved in zooming. This type of zoom lens can relatively easily provide a wide angle of view.
Japanese Patent Laid-Open Nos. 63-271214 and 64-72114 discuss zoom lenses including three lens units that have specific lens configurations and are moved under predetermined conditions in zooming. These zoom lenses include eight or nine lenses, have an angle of view of about 62° at a wide-angle end, and can successfully correct and/or reduce variations in aberration due to zooming.
According to the two publications, each of the lens units includes at least two lenses to achieve a higher power and thus reduce the total lens length (the distance from the first lens surface to an image plane) while correcting and/or reducing chromatic aberration within the lens unit itself.
Japanese Patent Laid-Open No. 4-153613 discusses a zoom lens including three or four lenses in total and having an angle of view of about 62° at a wide-angle end.
Japanese Patent Laid-Open No. 4-93812 discusses a zoom lens including four lenses in total and having an angle of view of about 58° at a wide-angle end. The zoom lenses above include three lens units, each including one or two lenses. The lenses of the lens units can be arranged substantially symmetrically during zooming and are formed of appropriate materials. The three lens units can thus reduce the total lens length while correcting and/or reducing aberrations including chromatic aberration with a good balance between the three lens units.
Japanese Patent Laid-Open No. 5-323190 discusses a zoom lens including three lenses that have appropriate shapes and can be arranged appropriately in terms of refractive power (the reciprocal of focal length) to reduce the amount of movement of each lens unit in zooming. This zoom lens can achieve an angle of view of about 74° at a wide-angle end with the three lenses.
Japanese Patent Laid-Open No. 7-120677 discusses a zoom lens including five lenses in total and having a first lens unit with a lower negative refractive power and a third lens unit with an appropriate negative refractive power. This zoom lens can successfully correct and/or reduce variations in chromatic aberration due to zooming and achieve a wider angle of view of about 73° at a wide-angle end with the five lenses.
Japanese Patent Laid-Open No. 2000-267002 discusses optical systems whose magnification changes with object distance, such as endoscope objective optical systems. A compact optical system according to an embodiment described in the publication includes fewer lenses and can achieve an angle of view exceeding 130° at a wide-angle end.
In addition, the optical system includes lens units arranged appropriately in terms of refractive power to inhibit and/or reduce variations in astigmatism due to the movement of a second lens unit, which can have a positive refractive power. These lens units can thus reduce the total lens length and achieve a wider angle of view.
An optical system including five lenses according to another embodiment of the reference causes about 50% of negative distortion to achieve an angle of view of 130° or more at a wide-angle end.
Japanese Patent Laid-Open No. 2000-330024 discusses a zoom lens for endoscope objective optical systems. This zoom lens includes five or six lenses and provides an angle of view exceeding 120°. According to this publication, the zoom lens causes about 50% of negative distortion at a wide-angle end to achieve an angle of view of about 122° with the five or six lenses.
Japanese Patent Laid-Open No. 3-4217 discusses real-image finder optical systems with variable magnification including three to five lenses. A real-image finder with variable magnification according to an embodiment described in the publication includes five lenses and has an angle of view of about 58° at a wide-angle end.
This system includes a second lens unit, which can have a positive refractive power to serve as the main component for changing magnification, and a third lens unit, which can have a low negative refractive power to correct and/or reduce the shift of an image plane due to changes in magnification. The third lens unit is composed of a single lens to reduce the size of the entire system.
In this system, a field frame is disposed inside an erecting prism to compensate for the disadvantage of a long back focus due to the low power of the third lens unit.
Japanese Patent Laid-Open No. 4-179908 discusses a real-image finder with variable magnification including four lenses and having an angle of view of about 56° at a wide-angle end. This finder includes a first lens unit, a second lens unit that is moved to change magnification, and a third lens unit, which can have a low power to correct and/or reduce the shift of an image plane due to changes in magnification.
The first and second lens units can be arranged appropriately in terms of power and include aspherical lens surfaces to successfully correct aberrations such as spherical aberration with the four lenses.
Japanese Patent Laid-Open No. 8-190132 discusses a Keplerian finder with variable magnification including three lenses and having an angle of view of about 51° at a wide-angle end. The power of each lens unit is appropriately adjusted to achieve an angle of view of about 51° at a wide-angle end with the three lenses.
Japanese Patent Laid-Open No. 11-249201 discusses a zoom finder including three or four lenses. According to this publication, the zoom finder includes a combination of glass and plastic lenses to achieve an appropriate diopter over a wide temperature range with high magnification.
Recently, the market for zoom lenses including fewer lenses and having a smaller total lens length, for example for cameras for use in cellular phones and compact digital cameras to achieve the size reduction of the entire apparatus, has increased. In addition, the demand for zoom lenses having wider angles of view, particularly over 80°, have also increased.
In general, a lens unit including more lenses is longer along the optical axis, and a zoom lens including lens units having larger amounts of movement in zooming has a longer total lens length. Thus, it is difficult to reduce the size of the entire lens system in such a zoom lens.
A negative-lead zoom lens including a frontmost lens unit having a negative refractive power is useful as a wide-angle zoom lens. It can be useful for this type of zoom lens to arrange lens units appropriately in terms of refractive power. If the arrangement of the lens units is inappropriate in terms of refractive power, the zoom lens has difficulty in achieving excellent optical performance over the entire range of magnification with a wider angle of view using fewer lenses and a simpler lens structure.